1. Field of the Invention
The present invention relates to a display apparatus including light-emitting elements and a method of producing the same.
2. Description of the Related Art
An organic electroluminescent element utilizing electroluminescence (hereinafter referred to as “EL”) of an organic material includes a first electrode and a second electrode disposed so as to face each other, and an organic layer disposed between the electrodes. Such an organic EL element has attracted attention as a light-emitting element that can realize high-luminance light emission by low-voltage driving.
An active matrix display apparatus (organic EL display) utilizing such an organic EL element includes thin-film transistors (hereinafter referred to as “TFTs”) corresponding to respective pixels on a substrate. Organic EL elements are arranged on an interlayer insulating film provided so as to cover the TFTs. Each of the organic EL elements includes a first electrode connected to a TFT and formed by patterning on the pixel, and an insulating element isolation film covering the periphery of the first electrode so as to leave the central portion of the first electrode exposed as a pixel opening. Each of the organic EL elements further includes an organic layer provided on the first electrode in the pixel opening, which is defined by the element isolation film, and a second electrode covering the organic layer. Among these, the second electrode is generally formed so as to cover a plurality of pixels and is used for the plurality of pixels in common.
In such an active matrix display apparatus, in order to ensure that a sufficient light-emitting area (aperture ratio) is provided in a single pixel area of the organic EL element, a top-face light extraction structure in which light is extracted from the top surface side of a substrate is effective. Accordingly, it is desired that the thickness of the second electrode be reduced in order to achieve sufficient light transmissivity. In such a case, the resistance of the second electrode increases, and thus voltage drop tends to readily occur.
To solve this problem, a structure has been suggested in which the voltage drop of such a second electrode is prevented by forming an auxiliary electrode made of a metal having satisfactory electrical conductivity on an element isolation film disposed between pixel openings to bring the auxiliary electrode into electrical contact with the second electrode.
As such a light-emitting apparatus, Japanese Patent Laid-Open No. 2002-318556 discloses a structure in which an auxiliary electrode is formed in the same layer as a first electrode, an organic layer is independently formed on each first electrode, and a second electrode is then formed so as to be in contact with the auxiliary electrode.
Furthermore, Japanese Patent Laid-Open Nos. 2003-316291 and 2007-92109 disclose methods in which a second electrode is formed, and auxiliary electrodes are then formed on the second electrode by vapor deposition using a shadow mask.
As described above, in the top-face light extraction structure, which is effective in an active matrix display apparatus, it is essential to increase the light transmittance of a second electrode, which is disposed on the light extraction side. In addition, in view of the effect of optical absorption loss and interference, it is necessary to form a thin second electrode. Consequently, the voltage may become nonuniform due to the in-plane resistance of the second electrode on the light extraction side, thereby degrading the display quality due to a decrease in the response speed, an increase in the electric power consumption, or the like.
To overcome this problem and to decrease the resistance of a second electrode, formation of an auxiliary electrode that is electrically connected to the second electrode has been proposed. However, in the structure disclosed in Japanese Patent Laid-Open No. 2002-318556, the auxiliary electrode is formed in the same layer as the first electrode, and thus the area of the first electrode is decreased by the area corresponding to the auxiliary wiring.
In addition, after the formation of an organic layer, it is necessary to bring the second electrode into contact with the auxiliary electrode, which is provided in the same layer as the first electrode. Therefore, it is necessary for a common layer contained in the organic layer to be aligned with high accuracy, the common layer being provided in common for each of colors, so that the common layer is not deposited on the auxiliary electrode. Accordingly, the structure of the apparatus may become complex.
As described in Japanese Patent Laid-Open No. 2003-316291, in the structure in which an auxiliary electrode is formed on an element isolation film to be in contact with a second electrode, the auxiliary electrode is formed by vapor deposition using a shadow mask in a direction of a long side of a pixel so as to extend to the outside of an image display unit on the element isolation film. Accordingly, the width of the auxiliary electrode may vary because of twisting and bending of the shadow mask in the direction of a long side of a pixel, and thus the production yield may be decreased. This phenomenon becomes more significant as the size of the screen of the display apparatus increases. As a result, it becomes difficult to arrange the auxiliary electrode in a non-light-emitting area.
To overcome this problem, according to the technique described in Japanese Patent Laid-Open No. 2007-92109, in order to maintain the strength of a shadow mask, a wiring pattern of the shadow mask is a broken line pattern having a selection ratio of 50%. Accordingly, auxiliary electrodes can be formed while preventing, for example, twisting of the shadow mask.
In this technique, however, after the auxiliary electrode formation is performed once, auxiliary electrodes are arranged in a broken line pattern. Therefore, vapor deposition must be performed again on areas where the auxiliary electrodes are not provided so that a film deposited in this step has the same thickness as the auxiliary electrodes formed above. In this case, a first alignment mark for forming first auxiliary electrodes and a second alignment mark for forming second auxiliary electrodes are provided in advance on the substrate side. After the formation of the first auxiliary electrodes, the shadow mask is shifted, and alignment is performed again using the second alignment mark.
After the formation of the first auxiliary electrodes by vapor deposition is completed at a first alignment position, the vapor deposition is performed on an area including an opening used for alignment of the shadow mask. Therefore, it is necessary that the second alignment mark be located in an area that is sufficiently distant from the first alignment mark.
For example, in the case where the position of the first alignment mark and the position of the second alignment mark are located within an area smaller than the opening used for alignment of the shadow mask, after the formation of the first auxiliary electrodes, the alignment mark used for forming the second auxiliary electrodes cannot be seen.
When the alignment marks provided on the substrate side are distant from each other by, for example, about 10 mm, it is necessary to shift the shadow mask by about 10 mm, accordingly.
That is, after alignment is performed in the formation of the first auxiliary electrodes, the shadow mask is shifted to the alignment position for forming the second auxiliary electrodes. Subsequently, alignment is again performed in order to form the second auxiliary electrodes. Accordingly, the process of forming the auxiliary electrodes is complex.